1. Field of the Invention
The present invention relates to an ink jet printing apparatus which print an image by moving a carriage that can mount an ink tank.
2. Description of the Related Art
A pigment ink has higher weatherability than a dye ink and is therefore beginning to be used on ink jet printing apparatus in recent years. The weatherability includes lightfastness, ozone resistance and waterfastness. Pigment particles contained in the pigment ink do not easily lose their color saturation if decomposed by light or ozone and their colors do not fade if exposed to light or ozone for a long period of time. Thus, the pigment ink shows its particularly excellent performance when used for the printing of outdoor advertisements and exhibits that are displayed for long periods or of pictures that need to be stored for long period. Further, since pigment particles are not water-soluble, the pigment ink has better waterfastness than dye ink. Because of these advantages, the oil-based pigment inks are widely used.
Generally, ink jet printing apparatus often use water-based ink. To make water-soluble those color materials not soluble in water, such as pigments, requires rendering the pigment particles hydrophilic by using polymer resins or surfactants and dispersing them in water or other solvent components.
If a pigment is used as an ink colorant and accommodated in an ink tank or other container and left unused, the pigment will deposit at the bottom of the ink tank, changing an ink concentration which will unavoidably become non-uniform. Solid particles such as pigment are suspended in a liquid as fine particles. If their specific gravity is greater than that of the solvent (medium), it is known from the following equation (1) that the particles will settle.u=2r2(ρ2−ρ1)g/9η  (1)
Where u is a settling rate of particles, r is a radius of particles when the pigment particles are assumed to be spherical, ρ1 is density of medium (solvent), ρ2 is density of particles, g is a gravitational acceleration and η is a viscosity coefficient of the medium. The equation (1) is called Navier-Stokes' equation. The equation (1) also shows that the water-based pigment ink with water as a main component of medium has a faster setting rate than that of the oil-based pigment ink. It is noted, however, that the pigment particles are subjected to a thermal motion of medium molecules in addition to the settling action by gravity and therefore are continuously in the Brownian motion. This Brownian motion causes the pigment particles to disperse, an action opposing the settling action, to realize a uniform particle distribution. This means that the pigment particles do not always settle according to the above equation (1). By improving the degree of hydrophilicity of pigment, i.e., the level of pigment diffusion in solvent, a pigment ink can be produced whose pigment particles will not easily deposit. However, the pigment particles still settle gradually in small numbers.
When the pigment particles settle, the property of the pigment ink, such as colorant concentration, viscosity and specific gravity, will change. In the ink jet printing apparatus, changes in a colorant concentration of the pigment ink lead directly to a color change in images, and changes in ink viscosity and specific gravity affect an injection volume and an injection speed of ink.
Therefore, in the ink jet printing apparatus using pigment inks, it is important to make the pigment ink concentration uniform in the ink tank.
As a method for making the pigment ink concentration in an ink tank uniform, it is known to directly stir ink in the ink tank. For example, Japanese Patent Laid-Open No. 4-087250 describes a method in which a reciprocally movable carriage mounting a print head and an ink tank is moved before a printing operation or at specified time intervals to agitate the ink in the ink tank. Another Japanese Patent Laid-Open No. 5-338195 discloses a method which rotates or reverses an ink tank by a motor to change the direction of gravity acting in the ink tank to prevent the sedimentation of pigment particles. Further, to ensure the effectiveness of the ink stirring, Japanese Patent Laid-Open Nos. 4-169240 and 9-309212 disclose a method that uses a steel ball in the ink tank to facilitate the stirring of ink.
If left unused for many hours, the color material in the ink in an ink jet printing apparatus settles, causing the ink concentration in an upper part of the ink tank to decrease and that of a lower part to increase, with the result that the ink concentration becomes non-uniform in the ink tank. If a recovery operation to maintain a good ink ejection performance of the print head is performed by discharging an ink, that does not contribute to the image printing, from a print head connected to the lower part of the ink tank, the high concentration ink is discharged from the ink tank. As a result, the colorant concentration in the ink tank gradually falls causing density variations in a printed image, which may lead to image impairments. The stirring operation, such as described in the above-cited references, can indeed alleviate the ink concentration changes in the ink tank or ink supply paths.
However, none of the patent documents cited above proposes a shortening of the time for stirring operation performed before the start of the printing operation.